Enantioselective ring-opening reaction of meso-epoxides with ArSeH catalyzed by heterometallic Ti-Ga-Salen system

Article abstract of DOI:10.1021/ol0503034

(Chemical Equation Presented) The first example of enantioselective ring-opening reaction of meso-epoxides with aryl selenols to give optically active β-arylseleno alcohol in up to 97% ee was realized, using a chiral Ti-Ga-Salen heterometallic catalyst. A strong synergistic effect of different Lewis acids in the system was exhibited in the catalytic process.

Full text of DOI:10.1021/ol0503034

ORGANIC
LETTERS
2
005
Vol. 7, No. 10
927-1930
Enantioselective Ring-Opening Reaction
of meso-Epoxides with ArSeH Catalyzed
1
by Heterometallic Ti
−Ga−Salen System
Minghua Yang, Chengjian Zhu,*,†,‡ Fang Yuan, Yijun Huang, and Yi Pan
†
†
†
†
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical
Engineering, Nanjing UniVersity, Nanjing 210093, China, and State Key Laboratory of
Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy
of Sciences, Shanghai 200032, China
cjzhu@netra.nju.edu.cn
Received February 14, 2005
ABSTRACT
The first example of enantioselective ring-opening reaction of meso-epoxides with aryl selenols to give optically active
â
-arylseleno alcohol
in up to 97% ee was realized, using a chiral Ti
system was exhibited in the catalytic process.
−Ga−Salen heterometallic catalyst. A strong synergistic effect of different Lewis acids in the
Selenium-based methods have developed rapidly over the
past few years and are now a very important tool in organic
synthesis. Chiral organoselenium compounds have been
pure substituted 1,3-oxazolidin-2-ones and tetrahydrofurans
starting from chiral â-arylseleno alcohols. It is of interest
to obtain the optically active â-arylseleno alcohols by the
catalytic asymmetric desymmetrization of meso-epoxides
with selenols, which has the advantage of simultaneously
establishing two contiguous stereogenic centers.
4
1
employed as useful ligands, catalysts, or intermediates in
various asymmetric transformations such as diethylzinc
addition to aldehydes, asymmetric hydrosilylation, and 1,4-
2
addition of Grignard reagents to enones. â-Arylseleno
The design and use of chiral homo- and heterobimetallic
complexes for asymmetric catalysis have seen significant
progress and are emerging as a rapidly developing area.5
Inspired by the advantage of enzymes that contain two or
more active sites, chemists have succeeded in developing
some kinds of multifunctional catalysts for asymmetric
alcohols are used as starting materials for several interesting
3
synthetic applications. Tiecco et al. recently described mild
and convenient methods for the synthesis of enantiomerically
†
Nanjing University.
Chinese Academy of Sciences.
‡
(1) (a) Pandey, G.; Gadre, S. R. Acc. Chem. Res. 2004, 37, 201. (b)
6
synthesis. For example, Shibasaki et al. developed a series
Wirth, T. Angew. Chem., Int. Ed. 2000, 39, 3740. (c) Khokhar, S. S.; Wirth,
T. Angew. Chem., Int. Ed. 2004, 43, 631. (d) Devan, N.; Sirdhar, P. R.;
Prabhu, K. R.; Chandrasekaran, S. J. Org. Chem. 2002, 67, 9417. (e)
Engman, L.; Gupta, V. J. Org. Chem. 1997, 62, 157. (f) Nakamura, S.;
Aoki, T.; Ogura, T.; Wang, L. B.; Toru, T. J. Org. Chem. 2004, 69, 8916.
of heterobimetallic, two-center Lewis acid-Br o¨ nsted base
asymmetric catalysts, in which the Br o¨ nsted basicity pro-
motes the reactions, while the Lewis acidity simultaneously
(
2) (a) Tiecco, M.; Testaferri, L.; Santi, C. Org. Lett. 2004, 6, 4751. (b)
Wirth, T. Tetrahedron Lett. 1995, 36, 7849. (c) Hiroi, K.; Suzuki, Y.; Abe,
I. Tetrahedron: Asymmetry 1999, 10, 1173. (d) Fukuzawa, S. I.; Tsudzuki,
K. Tetrahedron: Asymmetry 1995, 6, 1039. (e) Kazuki, O.; Yoshiaki, N.;
Sakae, U.; Akio, T. Tetrahedron Lett. 2004, 45, 6137.
(4) (a) Tiecco, M.; Testferri, L.; Temperini, A.; Bagnoli, L.; Marini, F.;
Santi, C. Chem. Eur. J. 2004, 10, 1752. (b) Tiecco, M.; Testferri, L.; Bagnoli,
L.; Purgatorio, V.; Temperini, A.; Marini, F.; Santi, C. Tetrahedron:
Asymmetry 2004, 15, 405.
(
3) (a) Tomoda, S.; Iwaoka, M. In Topics in Current Chemistry; Wirth,
(5) (a) Ma, J. A.; Cahard, D. Angew. Chem., Int. Ed. 2004, 43, 4566
and references therein. (b) Sammis, G. M.; Danjo, H.; Jacobsen, E. N. J.
Am. Chem. Soc. 2004, 126, 9928. (c) Ohno, K.; Kataoka, Y.; Mashima, K.
Org. Lett. 2004, 6, 4695. (d) Luo, Z.; Liu, Q.; Gong, L.; Cui, X.; Mi, A.;
Jiang, Y. Angew. Chem., Int. Ed. 2002, 41, 4532.
T., Ed.; Springer: Berlin, 2000; p 55. (b) Tiecco, M. In Topics in Current
Chemistry; Wirth, T., Ed.; Springer: Berlin, 2000; p 7. (c) Tiecco, M.;
Testaferri, L.; Santi, C.; Tomassini, C.; Marini, F.; Bagnoli, L.; Temperini,
A. Angew. Chem., Int. Ed. 2003, 42, 3131.
1
0.1021/ol0503034 CCC: $30.25
© 2005 American Chemical Society
Published on Web 04/22/2005

1
0
assists the catalysis by controlling the position of the
electrophiles as well as activating them, and successfully
applied them to a variety of enantioselective transformations
monometallic “open” complexes 2 in high yields. Reactions
of 2 with another equivalent of Ti(OiPr) gave the hetero-
4
bimetallic complexes 3, which were directly used as chiral
catalysts. Using hexane as the solvent, we initially tested
the asymmetric ring-opening reaction of cyclohexene oxide
with selenophenol at 20 °C with 3a and 3b as the catalysts
(10 mol %), respectively. To our delight, the reaction gave
the â-phenylseleno alcohol in 85% ee and 96% chemical
yield with 3b as the catalyst, while only 9% ee was obtained
with 3a as the catalyst. To the best of our knowledge, this is
the first example of catalytic enantioselective ring-opening
reaction of meso-epoxide with selenol.
7
with excellent selectivities. Very recently, Belcokon and
V
IV
Kagan showed that a mixture of two chiral V and Ti
complexes resulted in the formation of a mixed complex that
exhibited catalytic properties derived from both homometallic
species.8
Herein we wish to report the enantioselective ring-opening
9
reaction of meso-epoxides with aryl selenols catalyzed by
a novel heterobimetallic system, Ti-Ga-Salen catalyst
system, in which two different Lewis acids activate and/or
direct the substrate and nucleophile, respectively, and
promote the asymmetric reaction synergistically.
The reaction conditions were then optimized as shown in
Table 1 with 3b as a catalyst. From the results, we could
Scheme 1. Formation of Ti-Ga-Salen Heterobimetallic
Table 1. Asymmetry Ring Opening of Cyclohexene Oxide
with Selenophenol under Different Conditions
Catalysts
a
b
entry 3b (mol %) solvent T (°C) time (h) yield % ee %
1
2
3
4
5
6
7
8
9
0
10
10
10
10
10
10
10
5
Et2O
20
20
20
20
0
-40
-78
-40
-40
-40
2
2
2
2
3
5
5
5
5
5
96
90
93
96
96
94
83
93
92
62
69
35
83
85
90
97
91
97
95
89
CH2Cl2
toluene
hexane
hexane
hexane
hexane
hexane
hexane
hexane
2
0.5
1
a
Isolated yield after purification by prepared TLC. b Determined by
HPLC analysis on a Chiracel OD-H chiral column.
The readily obtainable Salen compounds (R,R)-1 were
chosen as the requisite ligands. As shown in Scheme 1,
treatment of 1 with an equivalence of GaMe provided
3
see that all of the solvents gave excellent chemical yields;
toluene and hexane provided good enantioselectivities (en-
tries 3 and 4), while Et O and CH Cl showed only mediocre
2 2 2
ee values (entries 1 and 2). Among the solvents tested,
hexane was proven to be the best solvent in terms of both
the chemical yield and enantioselectivity. A variation of the
reaction temperature from 20 to -40 °C caused a significant
increase in the ee value to 97% (entry 6), but there was a
small decrease when the reaction was carried out at -78 °C
(
6) (a) Gnanadesikan, V.; Horiuchi, Y.; Ohshima, T.; Shibasaki, M. J.
Am. Chem. Soc. 2004, 126, 7782. (b) Trost, B. M.; Mino, T. J. Am. Chem.
Soc. 2003, 125, 2410. (c) Trost, B. M.; Terrell, L. R. J. Am. Chem. Soc.
2
003, 125, 338. (d) Ooi, T.; Takahashi, M.; Yamada, M.; Tayama, E.;
Ohnoto, K.; Maruok, K. J. Am. Chem. Soc. 2004, 126, 1150.
7) (a) Shibasaki, M.; Kanai, M.; Funabashi, K. Chem. Commun. 2002,
(
1
989. (b) Shibasaki, M.; Yoshikawa, N. Chem. ReV. 2002, 102, 2187. (c)
Funabashi, K.; Jachmann, M.; Kanai, M.; Shibasaki, M. Angew. Chem.,
Int., Ed. 2003, 42, 5489. (d) Matsunaga, S.; Yoshida, T.; Morimoto, H.;
Kumagai, N.; Shibasaki, M. J. Am. Chem. Soc., 2004, 126, 8777.
(entry 7). We were pleased to find that there was no
significant change in enantioselectivity and chemical yield
when the catalyst amount was decreased to 5 mol % (entry
(8) Belokon, Y. N.; North, M.; Maleev, V. I.; Voskoboev, N. V.;
Moskalenko, M. A.; Peregudov, A. S.; Dmitriev, A. V.; Ikonnikov, N. S.;
Kagan, H. B. Angew. Chem., Int. Ed. 2004, 43, 4085.
8
). The reaction still showed good results even with only 2
(9) (a) Pastor, I. M.; Yus, M. Curr. Org. Chem. 2005, 9, 1. (b) Zhou, H.
Y.; Campbell, E. J.; Nguyen, S. T. Org. Lett. 2001, 3, 2229. (c) Matsunaga,
S.; Das, J.; Roels, J.; Vogl, E. M.; Yamamoto, N.; Iida, T.; Yamaguchi,
K.; Shibasaki, M. J. Am. Chem. Soc. 2000, 122, 2252. (d) Denmark, S. E.;
Barsanti, P. A.; Wang, K. T.; Stavenger, R. A. J. Org. Chem. 1998, 63,
mol % catalyst used (entry 9). Those results demonstrated
that the Ti-Ga-Salen heterobimetallic catalyst was very
efficient for the reaction. By the way, the addition of 4 Å
molecular sieves did not improve the reactivity and enan-
tioselectivity for the reaction.
2
428. (e) Cole, B. M.; Shimizu, K. D.; Krueger, C. A.; Harrity, J.; Snapper,
M. L.; Hoveyda, A. M. Angew. Chem., Int. Ed. Engl. 1996, 35, 1668. (f)
Jacobsen, E. N. Acc. Chem. Res. 2000, 33, 421. (g) Schneider, C.; Sreekanth,
A. R.; Mai, E. Angew. Chem., Int. Ed. 2004, 43, 5691. (h) Zhao L. S.; Han,
B.; Huang, Z. L.; Miller, M.; Huang, H. J.; Malashock, D. S.; Zhu, Z. L.;
Milan, A.; Robertson, D. E.; Weiner, D. P.; Burk, M. J. J. Am. Chem. Soc.
Asymmetric ring openings of a variety of meso-epoxides
with selenophenol and 1-selenonaphthol were investigated
2
004, 126, 11156. (i) Bartoli, G.; Bosco, M.; Carlone, A.; Locatelli, M.;
Massaccesi, M.; Melchiorre, P.; Sambri, L. Org. Lett. 2004, 6, 2173.
(10) Hill, M. S.; Wei, P.; Atwood, D. A. Polyhedron 1998, 17, 811.
Org. Lett., Vol. 7, No. 10, 2005
1928

Table 2. Ring-Opening Reactions of Different meso-Epoxides
with Selenophenol and Selenonaphthol Catalyzed by Different
Types of Catalystsa
for both cyclic and acyclic meso-epoxides, with catalyst 3b
giving the best chemical yields. In terms of enantioselectivity,
the open-chain mono- and homobigallium catalysts 2b
(entries 2 and 6) and 5 (entries 4 and 8) gave poor results,
while the cyclic titanium complex 4 provided moderate
selectivity (entries 3 and 7). As anticipated, the heterobime-
tallic catalyst 3b gave the highest ee values with the
substrates examined. The selectivity of catalyst 3b showed
significant margins over that of catalysts 2b, 4, and 5. In
other aspects, cyclic meso-epoxides provided better selectivi-
ties compared to acyclic ones, while the nucleophile sele-
nophenol gave better results than selenonaphthol. It is also
worth noting that the results represent a considerable
improvement with respect to the reaction promoted by thiols
1
2
and catalyzed by complex 4.
Although the complete reaction mechanism is not clear at
this moment, the results shown above manifest that the two
different Lewis acids Ti and Ga should work synergistically
in the heterometallic catalyst system. A possible working
model is depicted in Figure 1. It is likely that the epoxide
Figure 1. Possible working model for the synergistic effect (other
groups omitted for clarity).
a
Reaction conditions: epoxide (1.0 mmol), ArSeH (1.2 mmol), catalyst
b
c
(
5 mol %), hexane (3 mL), -40 °C, 5 h. Isolated yields. Enantiomeric
coordinates to the hard Lewis acid titanium metal and is
activated, while the nucleophile selenophenol coordinates to
the relatively soft Lewis acid gallium, which directs the attack
of the selenophenol to the epoxide more efficiently and
selectively. The coordination of the oxygen in the isopro-
purity was determined by HPLC analysis on a Chiracel OD-H column. The
determination of absolute configuration was based on the comparison of
CD spectroscopy of the product with that of sulfur analogue with known
1
2
absolute configuration.
1
under the optimized reaction conditions as in Table 1, entry
poxyl group to the gallium is supported by H NMR study.
11
8
. For comparison, Salen titanium complex 4, monogallium
The chemical shifts of the dimethylgallium hydrogens of
complex 3b appear at a higher field (δ -0.25 and -0.36
ppm) in comparison with that of complex 2b (δ -0.22 and
1
0
complex 2b, and homobimetallic gallium complex 5 were
also examined as the catalysts for the ring-opening reactions
of cyclohexene oxide and cyclopentene oxide with sele-
nophenol under identical conditions. The results are shown
in Table 2.
(11) (a) Belokon, Y.; Ikonnikov, N.; Moscalenko, M.; North, M.; Orlova,
S.; Yashkina, L. Tetrahedron: Asymmetry 1996, 7, 851. (b) Jiang, Y.; Gong,
L.; Feng, X.; Hu, W.; Pan, W.; Li, Z.; Mi, A. Tetrahedron 1997, 53, 14327.
From Table 2, it can be found that all reactions proceeded
smoothly to afford the â-arylseleno alcohols in high yields
(
12) Wu, J.; Hou, X. L.; Dai, L. X.; Xia, L. J.; Tang, M. H.
Tetrahedron: Asymmetry 1998, 9, 3431.
Org. Lett., Vol. 7, No. 10, 2005
1929

-
0.31 ppm). The coordination of the oxygen in the isopro-
respectively, and work synergistically. Other reactions
catalyzed by these kinds of heterometallic catalysts are now
under investigation in our group.
poxyl group to the gallium, which results in the close distance
between the titanium and the gallium and the formation of
a six-member transition state as well, is possible a crucial
factor.
In conclusion, the enantioselective ring-opening reaction
of meso-epoxides with aryl selenols to give the optically
active â-arylseleno alcohols was realized in up to 97% ee,
employing a novel type of chiral Ti-Ga-Salen heterome-
tallic system that contains two different Lewis acids. A strong
synergistic effect of different Lewis acids was exhibited in
the catalysis process. In this catalyst system, the two Lewis
acids activate and/or direct the substrate and nucleophile,
Acknowledgment. We gratefully acknowledge the Na-
tional Natural Science Foundation of China (20332050,
20472028) and the Science Foundation of Zhejiang Province
(M203052) for their financial support.
Supporting Information Available: Experimental pro-
cedures and characterization data. This material is available
free of charge via the Internet at http://pubs.acs.org.
OL0503034
1930
Org. Lett., Vol. 7, No. 10, 2005